Form 3853 (Health Coverage Exemptions and Individual Shared Responsibility Penalty)

I'm filling out Form 3853 (Health Coverage Exemptions and Individual Shared Responsibility Penalty) and having trouble with the Exemption Certificate Number field. I'm trying to indicate that I don't have an ECN, but the form won't let me proceed. I've tried entering 'NO ECN' but it's not accepting this entry. What is the correct format or text I should enter in this field to indicate I don't have an ECN? This is for tax year 2024. Is this a system error?

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